A successful and widely used method of manufacturing integrated circuits employs an optical system and a glass mask. Deposited on one surface of the glass mask is a circuit pattern. The mask is inserted into the projection machine. The optical projection system of the machine projects an image of the mask onto the surface of the silicon wafer suitably prepared as, for example, by application of a coat of photoresist. Examples of such instruments are the "Micralign" projection mask alignment and exposure systems manufactured by Perkin-Elmer Corporation of Norwalk, Conn.
Great care must be taken to keep the mask as clean as possible. Even a single speck of dust may cause substantial difficulty. It will be readily understood that dust may easily be picked up by the mask during storage or when transported back and forth between storage and the instrument. Accordingly, it is a primary object of the present invention to provide a system for maintaining a mask in a clean environment through storage and during transport into and out of the projection stage of the instrument. Other objects are to provide such a system wherein the complete operation of unloading the mask from the cassette and reloading it, as well as evacuating or pressurizing the cassette carrier, is accomplished automatically. Other objects, features, and advantages will be apparent from the following description and appended claims.